Tunable Upconversion And Luminescence Mechanism Of Ho³⁺-doped Core-Shell Nanoparticles

Rare-earth doped upconversion nanomaterials are characterized by narrow band emission,near-infrared excitation,long fluorescence lifetime and high photostability which can convert low-energy photons into high-energy photons,showing great potentials in biological imaging,therapy,display,lasing and information security.Among the rare-earth ions with upconversion emissions,Ho³⁺is an ideal luminescent center capable of producing the red and green upconversion as well as other functional properties such as magnetism.A seris of methods have been developed to accurately manipulate the Ho³⁺upconversion emission,such as the red dominated emission output by Ce³⁺doping and non-steady state upconversion,which contribute to the in-depth investigation of the of upconnersion mechanisms and frontier applications.Design of core-shell and conpenent control is beneficial to controlling spatial distributions of rare-earth ions and energy transfer for further regulating upconversion.Recenly,heavily doped upconversion systems have attracted a lot of attention for instance the Na Er F₄ materials has produced efficient upconversion of Er³⁺,promoting the development of high-concentration doped system research.By contrast,Ho³⁺doped latticle is limited by sensitization due to the lack of energy levels matching the wavelength of 980 or 808 nm.How to realize the upconversion in highly Ho³⁺doped systems,especially in the holmium lattice has remained a huge challenge.Moreover,there is lack of a systematic rearch on the Ho³⁺upconversion properties at low level doping with other rare earth ions.Thus,this thesis focuses on the spectral properties,upconversion mechanisms and the strategies of tuning emissions of holmium doped nanoparticles with different doping concentrations.The main results achieved in this thesis are as follows:（1）The Ho³⁺ upconversion porperties with co-doping of Eu³⁺,Tb³⁺,Dy³⁺,Sm³⁺and Ce³⁺were studied.Considering the spectral results and energy levels of the doping ions,it was found that Dy³⁺and Sm³⁺severely quench the upconversion emission of Ho³⁺by reducing the population of ⁵I₆ state while Eu³⁺and Tb³⁺mainly affect the ⁵I₇ state leading to a significant change in red to green emission intensity ratio.Then a core-shell structure of Na YF₄:Yb/Ho/Ce@Na Gd F₄:X based on UV-excitable properties of Ce³⁺is used to realize the downshifting emission in which Gd³⁺sublattice is characterized by energy migration.Moreover,this design can also respond to 980 nm wavelength excitation for red emission of Ho³⁺.Eventually,the dual mode emitting in the same nanoparticles provided a new approach for security and anti-counterfeiting application.（2）The tunable emission and upconversion mechanisms of high-concentration Ho³⁺doped lattice in a core-shell structure upon 980 and 808 nm excitation were systematically studied.First,the spectral properties of Na Ho F₄:Yb system was explored and it was found that the color output change from red to green can be realized by lifting Yb³⁺concentration.Then,a mechanistic core-shell design by separating Ho³⁺and Yb³⁺spatially in different shells using interfacial energy transfer was proposed and the spectral results comfirmed that it can effectively reduce deleterious back energy transfer processes from Ho³⁺to Yb³⁺and further improve the upconversion.Moreover,the red upconversion emission showed a dynamic dependence on the pulse duration and a gradual color change from red to yellowish green was achievable upon non-steady state excitation.Thus,the encoding and the decoding of hidden information was realized using this characteristic.By additionally introducing Nd³⁺in the outmost shell,it can also respond to the 808 nm excitation wavelength,and a gradual color change from red to green output was further realized through a smart control of Ho³⁺-Yb³⁺interactions in the interfacial region in the core-shell-shell structure by controlling Yb³⁺doping concentration.